Radiotherapy machines, such as the CLINAC machines manufactured by the assignee of the present invention, generally include a linear electron beam accelerator mounted on a gantry which rotates on an approximately horizontal axis. The electron beam accelerator is usually mounted on the gantry in such a manner that it is offset from the horizontal rotational axis of the gantry. The high energy electron beam emerging from the accelerator is further processed by techniques well-known to those experienced in the art to produce either an electron beam or an X-ray beam suitable for patient treatment. In either case the radiation is collimated in a treatment beam which is caused to travel in a direction perpendicular to the rotational axis of the gantry in such a manner that the axis of the treatment beam intersects the rotational axis of the gantry. The point at which the axis of the treatment beam intersects the rotational axis of the gantry is the focal point of the treatment beam and is referred to as the isocenter of the radiotherapy machine.
In a radiotherapy machine the patient is placed on a treatment couch that can be precisely positioned to locate the treatment region, which is usually a cancerous tumor or lesion in the patient, on the rotational axis of the gantry at the isocenter of the radiotherapy machine. Thus, by rotating the gantry, the source of the treatment beam can be rotated around the patient during treatment, thereby minimizing the amount of treatment radiation passing through any one region of the patient's body near the treatment region while the beam always passes through the treatment region itself. Excessive irradiation of non-diseased tissue, especially those tissues abutting the diseased treatment region, causes undesirable cell damage and cell death in healthy tissue.
Among practitioners of current radiotherapy treatment art it is well-known that minimum abutting cell damage generally occurs when the diseased treatment region in the patient is precisely located at the isocenter of the radiotherapy machine. However, several limitations of the present art make it difficult to achieve the desired precise positioning of the diseased region of the patient at the isocenter of the radiotherapy machine.
One reason for this difficulty is that diseased tissue in a patient usually is surrounded by, or is adjacent to, other soft tissue which is materially similar to the diseased tissue. The similarity of the tissues makes it difficult to precisely define the exact boundaries of the diseased tissue using current diagnostic and imaging techniques appropriate for radiotherapy machines.
One past attempt to overcome this problem has involved using relatively low contrast two-dimensional X-ray-based imaging of the region when the subject is positioned on the radiotherapy machine. The X-ray-based imaging systems have generally relied on detecting X-rays in the same X-ray beam which is used for radiotherapy purposes. However, low contrast two-dimensional X-ray-based imaging of the region does not enable the true position of the region including the tumor or lesion to be definitely located. The difference in X-ray absorbance between different soft tissue structures and between cancerous and non-cancerous soft tissues frequently ranges from small to undetectable. Only the bones, which absorb X-rays more strongly, can be readily imaged and precisely located by this means. Determining the true position of the soft tissue region to be treated is difficult because due to its lack of rigidity the region moves relative to the nearby bones of the subject as a result of unavoidable body movements of the subject on the treatment couch. The uncertainty in determining the true position of the region exists even when fiducial markers are inserted into the tumor because patient movement is likely to cause the fiducial markers to move.
Because the region desired to be treated is usually not located exactly as planned with respect to the isocenter of the radiotherapy system, insufficient quantities of radiotherapy beam energy are deposited in the region desired to be treated and excessive amounts of radiotherapy beam energy are deposited in healthy tissue in a volume abutting the region desired to be treated. Consequently, the tissue in the abutting volume is subjected to undesired and unnecessary damage so healthy organs adjacent the tumor site are damaged.
Because of the general inability to focus the radiotherapy beam with sufficient precision on the region desired to be treated, current medical practice is to increase the irradiated area to include additional tissue volume and to increase the dosage of the radiotherapy beam to ensure complete cell death in the region desired to be treated. The expectation is that all cells in the treated region are killed and possible positioning errors between the beam and the region are compensated. However, such techniques inevitably cause increased collateral radiation damage to the volume abutting the desired region to be treated, in some cases resulting in devastating quality of life effects on the subject. It is, accordingly, an object of the present invention to provide a new and improved method of, and apparatus for enabling a radiotherapy beam to be accurately positioned on a desired region to be treated by the beam.
Another object of the invention is to provide a new and improved method of, and apparatus for enabling a radiotherapy beam to be precisely positioned on a region desired to be treated, wherein the apparatus used to determine whether the beam is properly located is easily retrofitted on existing radiotherapy devices.
An additional object of the invention is to provide a radiotherapy machine including a magnetic resonance imaging system for acquiring 2D and 3D spatially resolved high-contrast images of soft tissue structures and organs within and abutting the region desired to be treated.
An additional object of the invention is to provide a radiotherapy machine including a magnetic resonance imaging system, wherein an excitation coil assembly of the imaging system is arranged so that a radiotherapy beam of the radiotherapy machine is not incident on the coil assembly and wherein the coil assembly is arranged so subjects to be treated can easily be placed in the path of the radiotherapy beam, on a treatment couch.
An additional object of the invention is to provide a new and improved radiotherapy machine in combination with a system for directly detecting the effect of the radiotherapy beam on an irradiated region, particularly the contents of tissue cells in the region, and to spatially resolve the effect of the irradiation to enable real time three-dimensional correlation between the shape, position and intensity of the region actually being irradiated and the known location of a region desired to be irradiated, where a tumor or lesion is located.
A further object of the invention is to provide a new and improved radiotherapy machine in combination with a relatively low cost device for determining whether, and the degree to which tissue in a region desired to be treated by a radiotherapy beam is actually being treated.
Still a further object of the invention is to provide a new and improved X-ray beam therapy device in combination with a magnetic resonance imaging system, wherein secondary electron skin dosage resulting from bombardment of the skin by the X-ray beam is substantially reduced by the magnetic field of the coils of the imaging system.